More specifically, the purpose of this invention is to improve the technology of pulverizing coal for burning in electric power generation boilers. This is done with a machine that is basically a system of spinning counter rotating rotors uniquely combined either with means for electrostatically and/or aerodynamically separating the fine pure coal from the pyritic and other impurities, with both separation and classification means, or with classification means only.
In U.S. Pat. No. 5,275,631, issued to Charles K. Brown and David K. Brown on Jan. 4, 1994, for a multicup dual counter rotating rotor centrifugal pulverizer, combined with purification means and classifier means, the object is to produce coal for burning with a lower sulfur content, thereby reducing the sulfuric acid emissions into the atmosphere, as is now being mandated by the government.
As chunks of coal are fed in through an axial center mounted feed tube, they are caused to smash repeatedly, at high velocity, onto other coal chunks and particles which have accumulated on the rings. By having the coal particles themselves act as the primary abrasion and reduction agents, material wear is minimized. Reduced in size from the series of abrasive collisions, the particles finally exit as an evenly dispersed circumferential spray of very fine material. At this point in the process, an in-stream aerodynamic and/or electrostatic separation action can readily be utilized to remove a high percentage of the sulfur and iron pyritic impurities contained therein.
Currently used pulverizing technology uses direct crushing means such as hammer mills, ball mills or roll mills of various configurations. In these mills, air is swept through the mill and as the coal is reduced to a fine enough size to be airborne the dust particles are entrained in the air stream and carried out of the mill to the combustor.
For material to leave the mill it has to stay in the mill until it is reduced to dust fine enough to become airborne by repeated crushing actions of the rolling or flailing elements of the mill. Pure coal and impure coal both leave the mill when ground fine enough to be swept up by the air currents blowing through the mill. Therefore, only limited separation of pure and impure coal takes place in these types of reduction mills.
When coal is mined, it often carries impurities mixed in its seams in the form of streaks ranging from small fractions of an inch to several inches in thickness. These stratified streaks of impurities are chiefly composed of both iron pyrites and sulfur, and when intermixed with the coal, comprise what is known as "bone" coal. Sulfur can also appear as chunks called "sulfur balls". Advanced coal cleaning methods remove a significant portion of this material, but a great deal of it remains with the coal. The bone coal is approximately three and one third times more dense and considerably harder than pure coal. Being harder, the bone coal requires greater energy to be reduced to dust in conventional mills. Yet, the mechanical crushing elements found in these types of mills do eventually reduce the bone coal to a fine enough size to be carried out to boiler burners by the air sweeping elements.
Thus, this conventional system of reduction offers a major drawback since the reduction of bone coal in these mills is not only useless, but the additional crushing power required to reduce the bone coal as well as the metal on metal contact produced therein results in high amounts of wear on mechanical parts. The present invention seeks, as one of its purposes, to use a means of reduction that will break down all or most of the coal passing through it to the extent that pyrites, ash-producing minerals and toxic elements (such as mercury and arsenic) can be separated from relatively pure coal by electrostatic means. The electrostatic means is effective only on particles 1/400 of an inch in size or smaller (-250 mesh).
It has been found that if the coal could be ground to a finer consistency (micronized) it would burn at a lower temperature and less Nitrogen Oxide (NOx) would be formed. Many available coals are low enough in sulfur content that the purification stages described above and embodied in U.S. Pat. No. 5,275,631 would not be required but if the end product leaving the pulverizer was fine enough the NOx problem would be helped. Therefore, another embodiment of the invention using a pulverizer to reduce the coal to a fine enough consistency combined with a size classifier to reject any remaining oversized chunks of coal coming out of the pulverizer would be effective in processing such low-sulfur coal. The oversized coal would be returned to the pulverizer for regrinding and the end results are greatly improved in the form of a higher percentage of finely ground coal product.
The construction and operation of apparatus and system will be described for pulverizing the coal. Also, two means will be shown for separating out the impurities. A further size classifying means will be discussed that will separate combustible size coal dust and oversize chunks that are returned to the mill for further reduction.
The use of this unique system of fuel preparation makes it possible to reduce operating costs of flue gas desulfurizers or where flue gas desulfurizers cannot be installed to markedly reduce sulfur dioxide emissions. The same mineral impurities in coal which contain iron and copper sulfites (pyrites) also contain combinations of ash-producing minerals and toxic elements which can be separated from the coal along with the pyrites. Further, the system can be used without separation means for pulverizing coal that has a low sulfur content.